Influence of the contact–impact force model on the dynamic response of multi-body systems

This work deals with contact–impact force models for both spherical and cylindrical contact surfaces. The incorporation of the friction phenomenon, based on the Coulomb friction law, is also discussed together with an effective computational strategy, which includes the automatic step size selection procedure. Impacts within a revolute clearance joint in a basic slider–crank mechanism are used as an example to compare the different contact force models. The collision is a prominent phenomenon in manymulti-body systems such as mechanisms with intermittent motion, kinematic discontinuities, and clearance joints. As a result of an impact, the values of the system state variables change very fast, eventually looking like discontinuities in the system velocities and accelerations. The impact is characterized by large forces that are applied and removed in a short time period. The knowledge of the peak forces developed in the impact process is very important for the dynamic analysis of multibody systems and it has consequences in the design process. The model for the contact–impact force must consider the material and geometric properties of the colliding surfaces, consider information on the impact velocity, contribute to an efficient integration, and account for some level of energy dissipation. These characteristics are ensured with a continuous contact force model, in which the deformation and contact forces are considered as continuous functions.FEDER - Project POCTI/2001/EME/38281.Fundação para a Ciência e a Tecnologia (FCT)

Ghost dynamics in the soft gluon limit

We present a detailed study of the dynamics associated with the ghost sector of quenched QCD in the Landau gauge, where the relevant dynamical equations are supplemented with key inputs originating from large-volume lattice simulations. In particular, we solve the coupled system of Schwinger-Dyson equations that governs the evolution of the ghost dressing function and the ghost-gluon vertex, using as input for the gluon propagator lattice data that have been cured from volume and discretization artifacts. In addition, we explore the soft gluon limit of the same system, employing recent lattice data for the three-gluon vertex that enters in one of the diagrams defining the Schwinger-Dyson equation of the ghost-gluon vertex. The results obtained from the numerical treatment of these equations are in excellent agreement with lattice data for the ghost dressing function, once the latter have undergone the appropriate scale-setting and artifact elimination refinements. Moreover, the coincidence observed between the ghost-gluon vertex in general kinematics and in the soft gluon limit reveals an outstanding consistency of physical concepts and computational schemes.Comment: 34 pages, 12 figure

Corneal Biomechanics in Ectatic Diseases: Refractive Surgery Implications.

BACKGROUND: Ectasia development occurs due to a chronic corneal biomechanical decompensation or weakness, resulting in stromal thinning and corneal protrusion. This leads to corneal steepening, increase in astigmatism, and irregularity. In corneal refractive surgery, the detection of mild forms of ectasia pre-operatively is essential to avoid post-operative progressive ectasia, which also depends on the impact of the procedure on the cornea. METHOD: The advent of 3D tomography is proven as a significant advancement to further characterize corneal shape beyond front surface topography, which is still relevant. While screening tests for ectasia had been limited to corneal shape (geometry) assessment, clinical biomechanical assessment has been possible since the introduction of the Ocular Response Analyzer (Reichert Ophthalmic Instruments, Buffalo, USA) in 2005 and the Corvis ST (Oculus Optikgerate GmbH, Wetzlar, Germany) in 2010. Direct clinical biomechanical evaluation is recognized as paramount, especially in detection of mild ectatic cases and characterization of the susceptibility for ectasia progression for any cornea. CONCLUSIONS: The purpose of this review is to describe the current state of clinical evaluation of corneal biomechanics, focusing on the most recent advances of commercially available instruments and also on future developments, such as Brillouin microscopy.(undefined)info:eu-repo/semantics/publishedVersio

Mudanças no diagnóstico pré-natal cromossómico: indicações clínicas, amostras biológicas, metodologias e cromossomopatias

Introdução: As mudanças no diagnóstico pré-natal de anomalias cromossómicas (DPN) nos últimos 10-15 anos foram contínuas e significativas. Objetivos: Propômo-nos analisar essa evolução: mudanças nas indicações clínicas; introdução das biópsias de vilosidades coriónicas (BVC); utilização do diagnóstico rápido de aneuploidias (DRA); estudos por microarray; alterações cromossómicas encontradas. Metodologia: Fez-se a avaliação retrospetiva nas gestações com amostras estudadas nos triénios 2004-2006 e 2014-2016. Analisaram-se os parâmetros indicação clínica, tipo de amostra, metodologias utilizadas e resultados. Resultados: Identificaram-se 68 fetos com cariotipo anormal em 2210 cariotipos (3,1%) em 2004-2006 e 208 fetos com cariotipo anormal em 2315 cariotipos (9,0%) em 2014-2016. A maior frequência de anomalias encontrou-se nos casos de rastreios ecográficos e combinados indicativos de risco acrescido de anomalia numérica e de progenitores portadores de alterações cromossómicas. As BVC permitiram respostas precoces nas gestações com anomalias numéricas e, adicionalmente, um aumento desses cariotipos (7.5% das amostras). O DRA permitiu ter uma resposta rápida nas anomalias numéricas mais frequentes (2 dias). As anomalias estruturais foram menos preponderantes nos cariotipos anormais (32,4% em 2004-2006 e 14.4% em 2014-2016). Discussão e conclusões: O DRA reduziu o tempo de resposta e das decisões sobre o futuro das gestações. O microarray permitiu identificar alterações sindromáticas em situações não resolúveis por outras metodologias. A utilização de BVC permite estabelecer uma melhor correlação fenotipo-genotipo em menores idades gestacionais. No entanto, as gestações com anomalias numéricas têm algum risco de perda fetal no primeiro e início do segundo trimestres. Assim, algumas BVC com cariotipos anormais resultariam em perdas espontâneas, o que poderia disponibilizar outros casos para DPN. Por outro lado, o menor número de anomalias estruturais equilibradas encontrado pode reduzir o conhecimento da variação genética nas famílias e na população. Um novo paradigma resulta da implementação dos testes não invasivos no DPN, para os quais ainda não conhecemos todas as limitações e repercussões.N/

Modelo experimental de lesão pulmonar aguda para estudo de manobras de recrutamento alveolar

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The state of art regarding scientists, outreach and evolution: a systematic literature review design

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Modelagem dinâmica do uso e cobertura das terras para o controle da erosão na bacia hidrográfica do Rio Mogi-Guaçú e Pardo – São Paulo – Brasil.

O objetivo deste artigo é a modelagem da dinâmica do uso e cobertura das terras na Bacia-Hidrográfica dos Rios Mogi-Guaçú e Pardo no Estado de São Paulo. Ele foca a restauração dos serviços ecossistêmicos do controle da erosão. São descritos e analisados a dinâmica do uso e cobertura das terras, focados nas suas principais forças direcionadoras, tais como a legislação ambiental do país e as mudanças nos preços relativos favorecendo uma forte expansão do cultivo da cana-de-açúcar na região. Esta dinâmica tem importante impacto nas taxas de erosão cujas medidas metodológicas são apresentadas com os resultados obtidos. Para simular esta dinâmica foi desenvolvido um modelo seguindo a metodologia do MIMES. O algoritmo desenvolvido modela as mudanças do uso e cobertura das terras baseado principalmente na força das leis ambientais. Espera-se que esta dinâmica contribua para a restauração dos serviços ecossistêmicos da Bacia Hidrográfica

A study on wear evaluation of railway wheels based on multibody dynamics and wear computation

The wear evolution of railway wheels is a very important issue in railway engineering. In the past, the reprofiling intervals of railway vehicle steel wheels have been scheduled according to designers' experience. Today, more reliable and accurate tools in predicting wheel wear evolution and wheelset lifetime can be used in order to achieve economical and safety benefits. In this work, a computational tool that is able to predict the evolution of the wheel profiles for a given railway system, as a function of the distance run, is presented. The strategy adopted consists of using a commercial multibody software to study the railway dynamic problem and a purpose-built code for managing its pre- and post-processing data in order to compute the wear. The tool is applied here to realistic operation scenarios in order to assess the effect of some service conditions on the wheel wear progression

Lubricated revolute joints in rigid multibody systems

The main purpose of this work is to present a general methodology for modeling lubricated revolute joints in constrained rigid multibody systems. In the dynamic analysis of journal-bearings, the hydrodynamic forces, which include both squeeze and wedge effects, generated by the lubricant fluid, oppose the journal motion. The hydrodynamic forces are obtained by integrating the pressure distribution evaluated with the aid of Reynolds’ equation, written for the dynamic regime. The hydrodynamic forces built up by the lubricant fluid are evaluated from the system state variables and included into the equations of motion of the multibody system. Numerical examples are presented in order to demonstrate the use of the methodologies and procedures described in this work.Fundação para a Ciência e a Tecnologia (FCT